1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a transflective type liquid crystal display device and method of fabricating the same that can improve picture quality.
2. Description of the Related Art
Generally, LCD devices are flat panel display device having advantages, such as a relatively small size, slim profile, and low power consumption. Accordingly, LCD devices are commonly used in mobile computers, such as notebook computers, office automation machines, and audio/video machines.
The LCD device displays images by manipulating transmission of light through a liquid crystal material having a dielectric anisotropy by controlling an electric field induced to the liquid crystal material. The LCD device makes use of an external light source such as a backlight or surrounding light. Thus, this LCD technique is in contrast to other display devices such as electro-luminescence (EL) devices, light emitting diode (LED) devices and the like, which emit light on their own.
The LCD devices may be classified, according to ways in which light is used, into two different categories: transmission type LCD devices and reflection type LCD devices.
A transmission type LCD device includes a liquid crystal panel having a liquid crystal layer interposed between two substrates, and a backlight unit supplying the light to the liquid crystal panel.
FIG. 1 is a view schematically showing a structure of a transmission type LCD device according to the related art. Referring to FIG. 1, the transmission type LCD includes: a lower substrate 102 having thin film transistors (TFTs) each functioning as a switching element formed at a crossing point of a plurality of gate lines and data lines; an upper substrate 101 facing the lower substrate 102 and having a black matrix (BM) layer, a color filter layer, and a common electrode formed thereon; a liquid crystal layer 103 interposed between the lower and upper substrates 102 and 101; a first polarizing plate 105 attached on the lower substrate 102; a second polarizing plate 104 attached on the upper substrate 101; and a backlight assembly 106 generating light and supplying the light towards the first polarizing plate 105.
In the related art transmission type LCD device having the foregoing construction, a TFT is turned on in response to a scanning signal, and accordingly, an angle of liquid crystal molecules of the liquid crystal layer 103 is changed by a potential difference between a data voltage supplied to a pixel electrode connected with the TFT and a common electrode supplied to the common electrode. Thus, by changing the electric field applied to the liquid crystal molecules having the dielectric anisotropy, the light supplied from the backlight assembly 106 is transmitted or blocked to display an image.
However, in the transmission type LCD device of the related art, it is difficult to realize slimness and lightweight of the LCD device due to a large volume and a heavy weight of the backlight assembly 106. Also, there is a problem that a power consumption of the LCD device is excessively increased because of the power consumption by the backlight assembly 106.
Therefore, researches for a reflection type LCD device not using the backlight assembly have been conducted.
The reflection type LCD device does not have a separate light source and displays an image using natural light (or surrounding light). Thus, since the reflection type LCD device does not need a separate backlight assembly, it has a low power consumption and accordingly is widely used as a portable display device, such as an electronic organizer or a personal digital assistant (PDA).
FIG. 2 is a view schematically showing a structure of the reflection type LCD device according to the related art. In FIG. 2, the reflection type LCD device includes: a lower substrate 202 having thin film transistors (TFTs) functioning as switching elements formed on crossing points of a plurality of gate lines and data lines; an upper substrate 201 facing the lower substrate 202 and having a black matrix (BM) layer, a color filter layer, and a common electrode formed thereon; a liquid crystal layer 203 interposed between the lower substrate 202 and the upper substrate 201; a first polarizing plate 205 attached on the lower substrate 202; a second polarizing plate 204 attached on the upper substrate 201; and a reflection plate 206 disposed below the first polarizing plate 205 and reflecting an external light towards the second polarizing plate 204.
In the related art reflection type LCD device having the foregoing construction, a TFT is turned on in response to a scanning signal, and accordingly, an angle of liquid crystal molecules of the liquid crystal layer 203 is changed by a potential difference between a data voltage supplied to a pixel electrode connected with the TFT and a common electrode supplied to the common electrode. Thus, by changing the electric field applied to the liquid crystal molecules having the dielectric anisotropy, the natural/surrounding light reflected by the reflecting plate 208 is transmitted or blocked to display an image.
In the reflection type LCD device having the foregoing construction, when a plurality of TFTs are turned on by a scanning signal applied to a plurality of gate lines, a predetermined data signal is applied to pixel electrodes through the turned-on TFTs. At this time, a common voltage is supplied to the common electrode of the upper substrate 201. Accordingly, the liquid crystal molecules are controlled by the electric field generated between the pixel electrodes and the common electrode to transmit or block light provided and reflected from the outside, whereby a predetermined image is displayed.
However, in the related art reflection type LCD device, when natural light dose not have a sufficient intensity (for example, when the surrounding is dark), the brightness level of a display image is lowered and the displayed information may not be readable, which is problematic.
Hence, a transflective LCD device employing both the advantages of the transmission type LCD and the reflection type LCD has been proposed.
FIG. 3 is a cross-sectional view schematically showing a construction of the transflective LCD device according to the related art. Referring to FIG. 3, the transflective LCD device includes: an upper substrate 310 that is a color filter substrate, a lower substrate 332 that is an array substrate, spaced apart by a predetermined interval from the upper substrate 310; a liquid crystal layer 320 interposed between the upper and lower substrates 310 and 332; and a backlight assembly 340 disposed below the lower substrate 332 and supplying light.
An upper polarizing plate 313 and a lower polarizing plate 336 are disposed on outer surfaces of the upper and lower substrates 310 and 332, i.e., on an upper surface of the upper substrate 310 and a lower surface of the lower substrate 332. The upper and lower polarizing plates 313 and 336 transmit only light parallel to their light transmission axes to convert natural light into a linearly polarized light.
The upper substrate 310 includes a transparent substrate 311, and a color filter and a common electrode 312 formed on the transparent substrate 311. The color filter transmits only a light having a specific wavelength, and the common electrode is supplied with a common voltage
The lower substrate 332 includes a transparent substrate 300. On the transparent substrate 300, a TFT is formed. A first passivation layer 334 having a transmission hole 331 is formed on the TFT. A reflection plate 335 is formed on the first passivation layer 334. A pixel electrode 333 is formed to be electrically connected with the TFT. In FIG. 3, reference numeral 332 is a second passivation layer formed to isolate the pixel electrode 333 from the reflection plate 335.
A gate line and a data line are disposed perpendicularly crossing each other on the lower substrate 332 to define a pixel region ‘p’.
The pixel region ‘p’ includes a reflection area ‘r’ and a transmission area ‘t’. The reflection area ‘r’ corresponds to the reflection plate 335, and the transmission area ‘t’ corresponds to the pixel electrode 333 positioned at the transmission hole 331.
Meanwhile, to reduce a difference in a distance where light travels between the transmission area ‘t’ and the reflection area ‘r’, it is designed such that a cell gap d1 of the transmission area ‘t’ is about twice larger than a cell gap d2 of the reflection area ‘r’.
Generally, a phase difference δ of the liquid crystal layer 320 is obtained by the following formula:δ=Δn·d where δ: phase difference of a liquid crystal, Δn: refractive index of a liquid crystal, d: cell gap.
Therefore, to reduce a difference in the optical efficiency between the reflection mode (which uses reflection of light) and the transmission mode (which uses the transmission of light), it is required that the cell gap d1 of the transmission area ‘t’ be greater than the cell gap d2 of the reflection area ‘r’ such that the phase difference value of the liquid crystal layer 320 is maintained constant.
However, although the cell gap d1 of the transmission area ‘t’ is larger than the cell gap d2 of the reflection area ‘r’, the optical efficiency in the reflection mode may be different from that in the transmission mode, which is problematic. In particular, when the reflection area ‘r’ and the transmission area ‘t’ are all formed within one pixel region ‘p’ and the transflective LCD device operates in both the reflection mode and the transmission mode, visibility may be lowered under a bright external light.
Also, since the process for forming the transmission area ‘t’ and the reflection area ‘r’ having different cell gaps d1 and d2 is very difficult and is complicated, a liquid crystal disclination may be caused due to a step height difference and a high process failure possibility.
Also, in the above structure, the brightness of the reflection area ‘r’ and the transmission area ‘t’ can be optimized. However, since the cell gap ‘d1’ of the transmission area ‘t’ is twice greater than the cell gap ‘d2’ of the reflection area ‘r’, the transmission mode and the reflection mode have a four times or more difference in the response rate, which is problematic.
In addition, since the cell gap of the transmission area is made twice greater than that of the reflection area, the overall thickness of the LC panel increases.